Electrophotographic image forming apparatuses are improved in various ways to meet requirements for higher speed image formation, higher quality image formation, full-color image formation and size reduction.
The key to the improvement is toner. That is, the toner is required to have smaller particle sizes, more uniform particle diameters and more spherical particle shapes in order to satisfy the requirements.
As for the smaller particle sizes, toners having an average particle diameter of not greater than 10 μm, more advantageously, an average particle diameter of not greater than 5 μm, have been developed. As for the more spherical particle shapes, toners having a sphericity of higher than 99% have been developed.
For further higher quality image formation, polymer toners supersede conventional ground toners. The polymer toners are excellent in dot reproducibility in image formation based on digital information and, therefore, advantageous for the higher quality image formation.
The image forming apparatuses employ a charging roller for uniformly electrically charging a surface of a photoreceptor body, and a developing roller for developing an electrostatic latent image formed by exposure of the electrically charged surface of the photoreceptor body into a toner image on the surface of the photoreceptor body.
The developing roller and the charging roller typically each include a roller body made of a crosslinking product of a rubber composition, for example, containing a rubber component and an electron conductivity imparting agent such as an electrically conductive carbon black, and a shaft such as of a metal inserted through a center through-hole of the roller body.
It is effective to use a semiconductive roller having a controlled roller resistance of not greater than 108Ω as the developing roller in order to efficiently develop the electrostatic latent image into the toner image while preventing the toner from adhering to the roller body. In addition, the semiconductive roller can impart the toner with higher electrical chargeability even if the toner is adapted for the smaller particle sizes, the more uniform particle diameters and the more spherical shapes or the polymer toner is used as the toner.
It is also effective to use the semiconductive roller having a controlled roller resistance as the charging roller in order to efficiently electrically charge the surface of the photoreceptor body with lower power consumption.
To meet various requirements imposed on the semiconductive roller, studies have been made on the type of the rubber component and the types and the proportions of additives for the rubber composition and the construction of the semiconductive roller.
For example, the roller body preferably has a nonporous single-layer structure in order to produce the semiconductive roller at higher productivity at lower costs and to improve the durability of the roller body and reduce the compression set of the roller body.
Where the semiconductive roller including the roller body having the nonporous single-layer structure is used as the developing roller, it is contemplated to use an ion conductive rubber such as a chloroprene rubber (CR) or an epichlorohydrin rubber as the rubber component in order to suppress reduction in toner charging amount to ensure higher quality image formation.
Where a semiconductive roller including a roller body made of a crosslinking product of a rubber composition containing the ion conductive rubber is actually used as the developing roller, however, a formed image is liable to have a reduced image density.
In Patent Literature 1, it is proposed that a filler (titanium oxide) having a toner adhesion preventing function is blended in the rubber composition in order to suppress the reduction in image density which may otherwise occur due to the adhesion of the toner to the roller body.
If the filler is blended in an amount sufficient to provide the effect, however, the roller body is liable to have an increased hardness to cause additional problems. More specifically, the roller body is liable to deteriorate the toner to reduce imaging durability, or liable to have a reduced nip width when being in press contact with the surface of the photoreceptor body to reduce the image quality of the formed image.
The term “imaging durability” is defined as an index that indicates how long the image formation quality can be properly maintained when the same toner is repeatedly used for the image formation. A very small part of toner contained in a developing section of the image forming apparatus is used in each image forming cycle, and the remaining major part of the toner is repeatedly circulated in the developing section. Since the developing roller is provided in the developing section and repeatedly brought into contact with the toner, whether or not the developing roller can reduce damage to the toner is a key factor to the improvement of the imaging durability.
If the imaging durability is reduced, a fogging phenomenon is liable to occur in a formed image. The fogging phenomenon is such that the deteriorated toner is spread over the background of the formed image to reduce the image quality.
It is also conceivable to allow the roller body to have a porous structure by blending a foaming agent in the rubber composition to impart the roller body with higher flexibility. However, the porous roller body has a shorter durable service life than the nonporous roller body and, therefore, is liable to suffer from permanent compressive deformation in a shorter period of time, thereby requiring replacement.
Patent Literature 2 proposes a semiconductive roller including a roller body having a double layer structure which includes an electrically conductive elastic layer, and a surface layer provided on an outer peripheral surface of the electrically conductive elastic layer, having a sea-island structure formed from a mixture of an acrylonitrile butadiene rubber (NBR) and a styrene butadiene rubber (SBR) immiscible with each other and containing an ion conductive agent.
Examples of the ion conductive agent include lithium perchlorate, sodium perchlorate, calcium perchlorate and perchlorates of long-chain-alkyl quaternary ammoniums.
Patent Literature 3 proposes a semiconductive roller including a roller body having a double layer structure which includes an elastic layer formed from a mixture of an ethylene propylene diene rubber (EPDM), an NBR and an SBR and containing an electrically conductive carbon black (electrically conductive carbon material), and a surface layer of a fluorine-containing material provided on an outer peripheral surface of the elastic layer.
Where the semiconductive roller is imparted with electron conductivity by using the electrically conductive carbon black alone as the electrically conductive agent, however, the roller resistance can be stabilized only by the layered structure including the surface layer covering the outer peripheral surface. That is, the roller body is not allowed to have a single layer structure, thereby requiring an increased number of production steps and an increased number of materials. Problematically, this correspondingly reduces the productivity of the semiconductive roller and increases the production costs.
Patent Literature 4 proposes a roller body formed from a rubber composition containing at least an SBR and an epichlorohydrin rubber as a rubber component. Patent Literature 4 states that the combinational use of the two types of rubbers provides the following effects:                increasing the toner charging amount when the roller body is used as the charging roller, and improving the toner imaging durability by imparting the roller body with flexibility to provide a proper nip width even if the roller body has a smaller roller diameter, and        suppressing the reduction in image density due to the adhesion of the toner to the roller body by reducing the amount of the epichlorohydrin rubber which may cause the adhesion of the toner.        